We propose to analyze at the cellular and subcellular level the processing of secretory IgA in the mammary gland of the rabbit along four main lines: 1. Localization of secretory IgA within both epithelium and plasmacytes in the mammary gland using immunoelectron microscope techniques. 2. Identification of sites of synthesis, pathway of intracellular transport and its metabolic requirements, and sites of combination of the subunits of secretory IgA by autoradiography, immunocytochemistry and cell fractionation procedures applied to short term and long term organ explant cultures. Identification of the mechanisms involved in cell to cell ( plasmacyte to epithelium) transport of secretory IgA. 3. Investigation of regulatory mechanisms (e.g. feedback controls, hormonal influence, and effect of antigenic stimulation) of secretory IgA processing in the same in vitro system. 4. Investigation of homing of lymphoid cells to the mammary gland and their differentiation into plasmacytes producing secretory IgA.